Silicon’s Superhero Cousin: Silicon Carbide
Updated: Nov 26, 2020
When choosing a semiconducting material for manufacturing electronic devices, silicon is by far the most popular option for several reasons. First and foremost, silicon forms high-quality silicon oxide, which provides insulation between various integrated circuit (IC) components. For this reason, silicon can be used to create a wide range of devices, from a simple diode to advanced microprocessors. Silicon is also used in ultra-low-power Bluetooth wireless connectivity devices and aids in switching power in industrial applications. While silicon has long been the semiconductor of choice in power electronics, it has its inherent limits as a power semiconductor due to its mechanical and electrical properties.
In the past ten years, demand for more energy-efficient chips has grown due to the emergence of a new computing era driven by the Internet of Things (IoT), big data, and artificial intelligence (AI). The general design philosophy is to reduce the size of transistors in such chips per Moore’s Law. However, advances in power semiconductors are not governed by lowering the node size but by breakdown voltage rating, on-resistance (or current rating), and thermal resistance. Silicon power devices are designed to handle voltages ranging from tens to hundreds of volts and hundreds of amps of current. As you can imagine, this is a lot of power handled by these devices! However, such power devices' capabilities made using silicon have limits, and these limits are driving the development of new semiconducting materials that offer superior performance.
Silicon carbide (SiC) is a compound semiconductor material containing silicon and carbon. Compared to silicon, an electron in SiC needs three times more energy to begin moving freely within the material. This wider bandgap gives the material interesting characteristics like faster switching and higher power density. This blog will highlight two use cases where SiC devices can deliver significant benefits in automotive electronics.
1. SiC in Automotive Electronics: Traction Inverters
A traction inverter is a machine that converts a DC power supply (i.e., battery) to an AC output, which will feed an electric motor. Inverters can be bulky and heavy if they require heat sinks or air cooling. This is usually the case for traditional silicon inverters, as they require those additions to mitigate their operating temperature.
Electric vehicles generally have a main motor powering the wheels, and it is common for six power transistors and diodes to be used to turn that motor. Each transistor must be able to block several hundred volts and switch a few hundred amps. In power applications, a key design criterion is to switch the device as quickly as possible to minimize wasted power and achieve higher efficiency.
Using SiC in a high-voltage design offers faster switching performances and higher power efficiency, translating to smaller and easier-to-cool modules. It is worth noting that when dealing with lower voltage rails, traditional silicon devices still have better performances, and their ubiquity indicates that they will remain common in the lower voltage (e.g., 12 V and 48 V) power electronic systems present in our vehicles. However, when dealing with systems operating at large voltages (e.g., 400 V, 800 V, or 1,200 V), the inherent properties of SiC enable new possibilities such as higher breakdown voltage, lower on-resistance, and thermal resistance. In an electric vehicle, the traction inverter takes a high voltage (typically between 400 V and 800 V) from the battery. It produces the three AC phases for the electric motor that will drive the vehicle.
Consider a situation where a silicon carbide traction inverter replaces a traditional silicon Insulated Gate Bipolar Transistor (IGBT) inverter. In Stockholm, a research group tested this replacement option on a local railway line to measure how a one-to-one replacement changes power efficiency. This configuration was performed twice to measure performance, first isolated in a laboratory and then on a Stockholm railway line. In the isolated laboratory test, the silicon carbide inverter replacement greatly reduced the increase of temperature over time by 63%. The laboratory test modeled the train application with a power source at 640 V, phase current 210 Arms, train speed 45 km/h, and the varying switching frequency at 1000 Hz. Although silicon carbide can operate beyond 1000 Hz, the laboratory test modeled a 1:1 replacement. At 1000 Hz, under identical conditions, the traditional silicon inverter switching losses were 88 Watts, and the silicon carbide inverter losses were 31 Watts. This tremendous improvement as the Silicon Carbide inverter reduced the switching losses by one-third, resulting in improved power conversion efficiency.
The field test increased the challenge. The train ran up to 70 km/h and needed up to 300 Arms in the field. The silicon carbide inverter replacement showed benefits in lower temperature rises and required a smaller protective case. The inverter varied between 10 and 20 degrees Celsius throughout the day, a variance of ~10-degrees. This is significant, considering that the traditional silicon inverter showed ~20-degree variance in the isolated laboratory test. The silicon carbide inverter in the field outperformed the silicon inverter even in an ideal laboratory situation.
This test helps understand the potential benefits in the electric vehicle industry since railway parallels the nature of start-and-stop traffic. Important takeaways of the improved traction inverter are the lighter physical weight and much-improved power efficiency.
The improvements showed a 22% weight reduction and a 51% volume reduction.
As an example, Tesla appears to be using silicon carbide in its inverter design. Tesla Model 3 is using silicon carbide MOSFET-based power modules from ST Microelectronics for its main inverter. They appear to operate at 650 Volts and 100 Amps, similar to the Stockholm railway test conditions.
The current trend in the industry indicates that SiC MOSFETs can help increase an electric vehicle’s range. These devices can drive the electric vehicle’s main motor with less power. Also, a higher switching frequency leads to higher power density and smaller, lighter motors. Furthermore, reducing wasted heat allows using smaller and lighter heat sinks, resulting in the optimization of the electric vehicle's weight and range.
As shown below, in the last ten years, China (371), Japan (354), and the US (351) appear to be the top three jurisdictions where major players in the automotive electronics industry have applied for patent protection on silicon carbide-based traction inverters. Mitsubishi Electric, Murata Manufacturing, and Hitachi have filed the highest number of patent families in this technology in terms of assignees. This indicates major Japanese players' business strategy in blocking competition in the US and Chinese markets, where Electric Vehicles are increasing in prominence.
Geographical Distribution of Patents for Traction Inverters - Top 10
(Source - Lumenci)
Top 10 Patent Assignees for Traction Inverters
(Source - Lumenci)
2. SiC in Automotive Electronics: Onboard and Offboard Chargers
SiC enables the size of the onboard charger and battery management solution of electric vehicles to shrink, which led to their integration onto the DC-DC converter and power distribution unit.
Three Standards of Chargers
The SiC switches are present in all level 1 – 3 charger types. However, only level 1 and level 2 switches are present onboard the car. The level 3 charger is an offboard converter since it is integrated with the charging station electric grid. Level 1 and 2 chargers are normally mounted within home garages or commercial spaces. These chargers function as safe extension cables to transmit power from the wall to the car. There is no power conversion performed on these wall units; instead, the power conversion from AC (Grid) to DC (Battery) is executed through the onboard charging unit residing within the vehicle. The onboard charger utilizes SiC switches to maintain high power efficiency and therefore fill the battery faster.
Level 3 charging units are large and add more cost to fit aboard the vehicle. Instead, charging station suppliers install the level 3 charging unit onto the station grid. Typically, the AC input is high voltage 480 V over 3-phases. The offboard unit uses SiC similar to that of level 1 and 2 chargers because it converts the AC input to a DC output. The difference is that the offboard unit circuit configuration is designed to accommodate a higher voltage and accepts all 3 phases.
As shown below, in the last ten years, major players in the automotive electronics industry appear to have filed patents related to silicon carbide-based onboard chargers in the CN (351), US (323), and EP (242) jurisdictions. Specifically, the top three assignees that filed the greatest number of patent families include Mitsubishi Electric, General Electric, and Witricity. It is interesting to note that Witricity is a relatively young company founded in 2007 by a team of scientists from the Massachusetts Institute of Technology (MIT).
Geographical Distribution of Patents for Onboard and Offboard Chargers - Top 10
(Source - Lumenci)
Top 10 Patent Assignees for Onboard and Offboard Chargers
(Source - Lumenci)
In conclusion, Silicon Carbide is an upcoming candidate to impact the Electric Vehicle industry. While we reviewed only a subset of Silicon Carbide applications in traction inverters, onboard chargers, and offboard chargers in this blog, the future looks very bullish for the impact of this technologically significant semiconducting material across a wide range of power electronic device applications too.
Senior Consultant at Lumenci
Karthik is Semiconductor Expert at Lumenci. He has experience in working with Nanotechnology. He holds a Ph.D. in Electrical Engineering from the University of Illinois at Urbana-Champaign.
Senior Associate at Lumenci
Celeny is a Wireless Charging Expert at Lumenci. She holds a degree in Electrical and Computer Engineering from the University of Texas, Austin.
Associate at Lumenci
Nupur is a VLSI Design Expert at Lumenci. She has experience in Semiconductor and Microelectronics Technology. Nupur holds a master's degree in VLSI Design from IIT Dhanbad.